1. Field of the Invention
This invention relates generally to measurement of cross-flow vortices and more specifically to a method of measuring the wavelength of cross-flow vortices by an array of hot-film sensors.
2. Description of the Related Art
The attainment of laminar air flow on aircraft wings has significant potential for reducing drag and increasing fuel efficiency. One method of drag reduction involves the avoidance of or delay in the transition of the laminar boundary-layer to turbulent flow. On a swept wing the air flow is highly three-dimensional, and four fundamental instability modes--contamination at the attachment line and streamwise, centrifugal, and cross-flow disturbances--strongly influence the transition of the laminar boundary-layer to turbulent flow.
Control of the cross-flow disturbances first requires understanding of its development in the laminar boundary-layer. Cross-flow disturbances can be characterized as a stationary disturbance with regularly-spaced vortices and a non-stationary or traveling disturbance having large amplitude harmonics. Cross-flow vortices arise from a dynamic or inviscid instability of the inflectional cross-flow velocity profile produced by the three-dimensionality of the mean flow field. Detailed measurements of the cross-flow vortices would facilitate an understanding of the cross-flow disturbances.
Prior art includes both experimental and theoretical studies on the nature of cross-flow disturbances. Poll discloses in "Some Observations of the Transition Process on the Windward Face of a Long Yawed Cylinder," J. Fluid Mech., Vol. 150, 1985, pp. 329-56, the effect of cross-flow instability upon the boundary-layer transition on a swept cylinder. Malik et al in AIAA 84-1672 (1984) theoretically studied the cross-flow over a yawed cylinder including curvature effects. Dagenhart et al in AIAA 89-1892 (1989) observed stationary cross-flow vortices through flow visualization using sublimating chemicals. Sari et al in AIAA 85-0493 (1985) made detailed hot-wire measurements in the spanwise and chordwise direction at a constant distance from the wing surface. Their hot-wire measurements at the maximum cross-flow velocity location resulted in a different wavelength of the cross-flow vortices from the wavelength obtained during flow visualization. Mangalam et al in AIAA 90-1636 disclose hot-film measurements at one chord location to obtain cross-flow characteristics.